Rolling mill control system

ABSTRACT

A rolling mill and associated control system are described, including means for adjusting roll spacing of the mill to keep the exit gage of rolled strip substantially constant as the mill is accelerated to normal operating speed; the adjusting means being operable selectively over a range of mill speeds to cause roll spacing adjustments in relation to subsequent changes in mill speed.

United States Patent [191 McKee et al.

[ Jan. 22, 1974 ROLLING MILL CONTROL SYSTEM [75] Inventors: James D. McKee; John F. Lamb,

both of Florence, Ala. [73] Assignee: Reynolds Metals (Iompany,

Richmond, Va.

[22] Filed: Apr. 12, 1972 [21] Appl. No.: 243,130

[52] US. Cl. 72/19 [51-] Int. Cl B2lb 37/00 [58] Field of Search 72/8, l1, 19, 21, 9

[56] References Cited UNITED STATES PATENTS 3,704,609 12/1972 Sterret 72/19 3,365,920 1/1968 Maekawa et a1 ..72/l

3,603,124 9/1971 Arimura et al. 72/8 3,128,630 4/1964 Briggs 73/432 3,045,517 7/1962 Wallace et al. 72/9 Primary ExaminerMilton S. Mehr Attorney, Agent, or Firm-John F. C. Glenn et al.

[5 7] ABSTRACT 12 Claims, 3 Drawing Figures SPEED FEEDBACK 50 r Q42 L 9 m1.

SPEED REGULATOR FEEDBACK (STRIP NETWORK oPERAToRs MAmAL REF. POSITION 3e 32 FEEDBACK FEEDBACK NETWORK FEEDBACK v v0 01mm ETWOR I I POSITION N K D REGULATOR 64 as vvvv oPERAToR's AUTOMATIC 40) REF.

REFERENCE 1K. VB A/D NSF) MODlFlER FROM I co vERrER m ROLLING MILL CONTROL SYSTEM This application concerns an improved control system for rolling mills, and related aspects of operating such mills. More particularly, the application discloses a roll positioning system for adjusting the roll spacing of a mill (as well as circuit means adapted for modifying conventional control systems) to achieve improved operation during acceleration of the mill, including more uniform exit gage of the rolled product and reduced scrap losses due to off-gage conditions. i

It is conventional in the rolling mill art to provide automatic gage control systems for varying either or both of the roll speed and the roll spacing in response to variations in exit gage of the resulting strip; also, for regulating the strip tension between roll stands of a multistand mill for like purposes. Such systems are generally effective for counteracting minor variations in thickness during production of strip at normal operating speeds of the mill. On the other hand, it has been difficult to provide a system capable of achieving satisfactory gage control during rapid acceleration of the mill, such as that occurring on startup and following low speed threading operations. Consequently, due to very high speeds involved in modern mill practices, upwards of 3,000 fpm in the case of typical multistand cold rolling mills, for example, a serious problem has been the generation of considerable off-gage material even during periods as short as or seconds commonly involved'in'bringing the mill up to speed.

This problem is caused in part by the well-known characteristic that metal being rolled between opposed work rolls tends to become thinner as the rolling speed increases. Other factors including bearing behavior can exaggerate or compound this speed effect, particularly in rolling mills using oil film bearings including those having a hydrostatic lubrication system. Thus, not only is there a thinning tendency of the metal due to its own rolling characteristic as a function of mill speed, even at constant roll spacing, but there can be a speed related change in the roll spacing itself due to oil film thickness variations of the bearings. Generally these effects are similar and cumulative, requiring correction or compensation by increasing the roll spacing in both instances as the mill speed increases.

One approach commonly used is to provide a programmed change of roll spacing, i.e., to anticipate and counteract at least approximately the expected thinning of the rolled strip as a function of mill speed. Changes in roll spacing are accomplished by providing drive means such as a motor and screwdown mechanism or a similar hydraulic system, and an associated control device for applying the predetermined roll spacing correction (sometimes called screw program).

Considerable operating experience with a commercially available system of this type has shown several shortcomings. The start-up operation of a rolling mill involves threading the metal through the various roll sets of the mill. while running at relatively low speed. This constitutes a generally unstable period of operation due to numerous variables, particularly when changing from one product to another as regards different input or output thicknesses and/or different alloys. It is a time when operator skill is most important and it involves a general tune-up of mill performance. Once the desired exit gage has been achieved, however, it then becomes essential to accelerate the mill to fullspeed production condition, preferably with the least possible deviation of product thickness.

When the commercial system previously referred to was operated continuously, commencing at zero mill speed, it necessarily caused progressive changes in roll spacing which affected exit gage of the rolled strip, thereby complicating operator adjustments during the critical start-up period. However, if the screw program was initiated after reaching on-gage condition at a higher speed, it caused a sudden speed related correction of roll spacing which changed exit gage of the strip. This necessitated further adjustments, with resulting increase in the amount of off-gage scrap generated.

Consequently, an objective of the present invention was to provide a screw programming control system for making speed related roll spacing adjustments, which could be actuated at any speed without causing disruption of gage control at the existing mill speed, yet would effectively provide roll spacing corrections during subsequent acceleration of the mill.

These and other objects, advantages and details of the invention will appear in connection with the following description of presently preferred embodiments and practices thereof, as illustrated in the accompanying drawings. FIG. 1 shows an overall schematic arrangement of one roll stand of a mill and the associated control system and its various components; FIG. 2 shows detailed circuitry of an improved subcomponent aspect of the system; and FIG. 3 shows representative criteria of roll spacing adjustments as a function of certain operating variables.

Referringfirst to FIG. 1, the mill includes a 4-high roll stand 10 having upper and lower work rolls l2, back-up rolls l4 and power driven screwdown position regulator 16. A force transducer (load cell) 18 adjacent the lower back-up roll measures roll force, i.e., generates an output voltage V,- proportional to the separating force exerted on the work rolls. The mill speed tachometer device 20 produces an output voltage V proportional to the speed of the work roll surface. Not shown are other conventional or repetitive elements of the mill including the roll drive system and its variable speed controls, interstand tension controls, payoff and winding reels, strip thickness gages and other routine mechanical or electrical components. The mill is typically a S-stand continuous cold mill, of which the mill stand 10 is representative.

The roll positioning system of FIG. 1 includes regulator l6 and its drive motor 22, variable voltage speed regulator 24, digital position regulator 26 and suitable feedback systems as shown, all of which are conventional. Provision is also made for manual input to both regulators, and the usual automatic gage control input 28 to regulator 26 is also shown. The present invention relates primarily to other portions of the system, particularly the means for producing or modifying the screw programming input 30 to the digital position regulator 26, as will now be described in greater detail.

The screw positioning control unit in certain respects follows conventional practice. Thus, a force amplifier system 32 is provided including input resistor 34, operational amplifier 36 and feedback network 38, to convert the output voltage V of load cell 18 (which represents roll force of mill stand 10) into an output signal of the linear form V, GV k. The latter signal constitutes one input to the analog digital converter 40.

Similarly, a speed amplifier system 50 is provided to convert the output voltage V of tachometer device (which represents the work roll speed of mill stand 10) into an out-put signal V, of the desired characteristic. As indicated in FIG. I, an illustrative speed amplifier characteristic is such as to provide an output signal of the form V =G,V +G (V K,) G (V k representing a theoretically or empirically determined function of roll spacing in relation to mill speed for substantially constant gage of the strip being rolled. Thus, for example, in the present system this relationship typically is based on graphical representation (see FIG. 3) of changes in oil film thickness of two bearings (upper and lower work rolls) as a function of roll speed, for various roll force conditions. Consequently, the output signal V serves to indicate for a particular roll force condition the appropriate roll spacing correction for any change in mill speed.

It will be appreciated, however, that the present invention is not limited to making roll spacing adjustments in relation to bearing behavior, but is adapted for controlling other speed-related aspects of mill operation in accordance with any selected criterion. Thus, for example, experience indicates that the rolling phenomenon of decreasing gage with increasing speed (for cold rolling aluminum and aluminum base alloys) follows generally the same pattern as that of oil film effects in bearings. Consequently, using a mill equipped with Morgoil oil film bearings, it has been found convenient to overcompensate for the bearing effect to an extent of increasing the roll spacing correction by about 75%, to take into account other speed related corrections needed to maintain substantially constant output thickness of the rolled strip while accelerating the mill. Furthermore, although the curve following feature has been illustrated with regard to the speed amplifier output, it will be recognized that similar effects may be achieved for other operational factors if desired.

Returning to the overall arrangement of FIG. 1, it is imporatant to note that the speed amplifier 50 is coupled to converter 40 by intermediate circuitry shown generally at 60, representing a significant departure from the conventional aspects previously described. This circuitry is detailed in FIG. 2. Its purpose and function is to enable the screw positioning program of predetermined changes in roll spacing to be initiated independently of mill speed, without causing undesirable screw adjustments in relation to prior mill speed changes. That is, in contrast to a conventional system which must be actuated at zero mill speed or some specific higher speed, the present system effectively excludes from consideration any prior speed changes and causes compensating roll spacing adjustments only in relation to mill speed changes occurring after the automatic mode of operation is selected. The advantage of this arrangement is that, regardless of what speed has been reached when the program commences, the roll spacing remains unchanged at that speed and no adjustment occurs which would change thickness of the rolled strip. Thus, the operator has complete flexibility under start-up conditions, can vary speed and other operational controls at will until stabilized operation exists for the desired output gage and may then, at whatever intermediate mill speed has been reached, select automatic screw programming mode and proceed to accelerate the mill to full speed condition. Any relatively wide variations in gage which occur are largely limited to periods of low speed operation, with correspondingly less off-gage scrap being generated. How this is accomplished may be seen from the following description relative to FIG. 2.

In the circuitry 60, output signal V, of the speed amplifier 50 is applied to amplifier unit 62 through input resistor 64. The corresponding linear output signal V of amplifier 62 is passed through normally closed contacts of relay 66 to actuate a motor driven potentiometer 68. The voltage V,, read from the latter device is used to drive a stiff amplifier unit 70 having input resistor 72 of the same value as its feedback resistor 74. Similarly, the balancing resistor 76 is equal to input resistor 64 of amplifier unit 62. The latter unit includes a feedback network 65 having Zener diodes Z and Z which clamp the amplifier components 0A1 and PAl to limit the voltage at the armature terminals of potentiometer motor 68 to the maximum rating of the motor.

When the system is balanced the potentiometer motor shuts off because the output voltage V,, of amplifier 70 is equal to (but of opposite polarity from) the input voltage V to amplifier 62. Also, the output resistor 78 (of speed amplifier 50) and output resistor 80 (of amplifier 70) are equal, thereby producing zero voltage input V to the A/D converter 40.

The result of this arrangement is to effectively null any output of the speed amplifier (and thus avoid any speed related input to the converter) until such time as the contacts of relay 66 are opened by the operator. Then the circuitry 60 through the memory behavior of the potentiometer 68, provides an initial reference condition of mill speed, and subsequent changes in speed relative to that reference condition result in corresponding changes in roll spacing. In other words, when contacts of relay 66 are opened (thus selecting screw programming on) the voltage V,,' is maintained at the value thereof present immediately before opening the contacts; and voltage V will vary thereafter in proportion to subsequent changes of input voltage V Referring to FIG. 3, data is presented in graphical form showing a selected function of the mills roll spacing (viz. the total oil-film thickness of two Morgoil bearings which support the work rolls) in relation to the mill speed for various roll force conditions. Entering this plot at 800 fpm, for example, as indicated on the drawing, it can be seen that the point of intersection with a particular force curve (the one representing a roll separating force of 2 million pounds) corresponds to a film thickness of nearly 0.007 inches. Thus, in reaching even a relatively low speed, the bearing behavior causes decreased roll spacing to a considerable extent. Unless compensated by a corresponding adjustment of the roll spacing this results in the rolled strip becoming thinner.

As previously discussed, preliminary roll spacing corrections are conveniently made under start-up conditions by manual control of the mill. When operating the mill in its automatic roll positioning mode, however, after reaching the desired strip thickness at a convenient intermediate speed, it can be seen by reference to FIG. 3 that each incremental speed change AS as the mill is accelerated to normal operating speed requires an additional compensating adjustment AR to keep the strip thickness substantially constant. The roll positioning system of the present invention is operable selectively to cause such adjustments in relation to subsequent changes in mill speed, thereby eliminating the effeet of prior speed changes. In a practicalsense, therefore, the circuit means previously described afford a variable speed reference condition which serves as the starting point for speed compensating adjustments as themill is accelerated in its automatic roll positioning mode, thus controlling incremental adjustments of roll spacing relative to corresponding changes in mill speed. This avoids sudden spurious adjustments due to speed changes occuring prior to reaching on-gage condition.

In summary, therefore, the mill operator can cause the roll spacing to change in accordance with a manual reference input to regulator 24 (FIG. 1), and can also regulate the mill speed in connection with preliminary start-up operations at relatively low speed, typically in the range of 500 to 1,000 fpm. Once on-gage operation 'has been achieved, the operator then selects automatic screw programming mode (by opening the contacts of relay 66), independently of the particular mill speed at that time, and the roll positioning system is effective to maintain exit gage of the rolled strip substantially constant as the mill is accelerated to normal operating speed. The system remainsresponsive also to the conventional automatic gage control (AGC) input 28 to position regulator 26, for modifying the mills roll spacing or interstand tension of the strip in accordance with' output gage of the metal strip ing variable resistor RH3 (0 20K ohm);

b. Components Q Al .and 0A2 of amplifier units 62 and 70 respectively are available together as .GE item IC30 QAA2,l. assi nm A is. Q? 7 te c. Components Z and Z of feedback network 65 are 6.2 volt'(l watt) Zener diodes of the Motorola HEP series (No. 103), and the network also includes resistor R3 (47K ohm) and capacitor Cl (0.15 mfd);

EL Resistor 64includes a fiited component RI (4.7K

ohm), and a variable component RHl (040K ohm) adjusted at about 5.3K ohm to give a total of K ohm equal to resistor 76;

e. Resistor 80 includes a fixed component R6 (2.7K ohm), and a variable component RH2 (0-lK ohm) adjusted at about 0.6K ohm to give a total of 3.3K ohm equal to resistor 78;

f. Resistors 72 and 74 are l meg-ohm size, and C2 is a 0.1 mfd capacitor;

g. Resistors R4, R9 and R10 are 1K ohm, 27K ohm and 12K ohm sizes respectively;

Meal/t s GE. i m cl omsascz t. The rest of the control system shown in FIG. 1 is a DIRECTOMATlC II unit of General Electric Company, and the rolling mill is a Bliss four-high (55 X 22 X 66) S-stand cold mill. The output signal of A/D converter 40 has the form N K (VB/KB) (Ko/Vo) which is a binary number representing an increment of screw movement (i.e., change of roll spacing).

Although presently preferred aspects of the invention have been illustrated and described, it will be appreciated that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

What is claimed is:

1. In a rolling mill having opposed work rolls for reducing the thickness of a metal workpiece passing therebetween and a roll positioning system for increasing the roll spacing automatically to compensate substantially for decreasing thickness of the workpiece as the mill speed increases, the improvement comprising:

control means which may be selectively rendered operable by an operator at any speed over a range of mill speeds for actuating said roll positioning system, said control means including,

means operable before and after said control means is rendered operable for generating an output signal representing a predetermined function of roll spacing in relation to mill speed, and

means for modifying said signal to eliminate the effectof changes in mill speed previous to the time said control means is rendered operable.

2. A rolling mill according to claim I wherein said signal generating means comprises means for sensing mill speed and amplifier means responsive to said sensing means for generating an output signal representing a function of roll spacing in relation to mill speed.

3. A rolling mill according to claim 2 wherein said means for modifying the output signal of said amplifier means comprises a closed loop servo system including a motor driven potentiometer and associated circuitry operable selectively to produce a modified signal representing said function of roll spacing relative to changes in mill speed subsequent to the time said closed loop servo system is rendered inoperable.

4. In a rolling mill having opposed work rolls for reducing the thickness of a metal workpiece passing therebetween, including roll positioning means for adjusting the spacing between said rolls and a control system for causing programmed changes in roll spacing as a function of roll force and mill speed, the improvement wherein-said control system comprises the combination of:

a. a load cell for sensing the force acting on said rolls;

b. a force amplifier unit responsive to said load cell for generating an output signal proportional to the roll force;

c. means for sensing mill speed;

d. a speed amplifier unit responsive to said speed sensing means for generating an output signal representing a predetermined function of roll spacing in relation to mill speed;

e. circuit means operable selectively in response to said output signal of the speed amplifier over a range of mill speeds to produce a modified signal representing that portion of the speed amplifier output caused by changes in mill speed subsequent to the time said circuit means is made operable; and

f. an analog-digital converter responsive to said modified signal and the output signal of said force amplifier for generating a position signal to actuate said roll positioning means.

5. In a rolling mill having opposed work rolls for acting on a metal workpiece passing therebetween, the thickness of said workpiece as it leaves the rolls being affected by the mill speed, becoming progressively thinner as mill speed increases, the combination of:

a. roll positioning means for adjusting the spacing of said rolls during operation of the mill; and

b. control means for actuating said positioning means to cause roll spacing adjustments effective to keep exit thickness of the metal substantially constant as the mill is accelerated to normal operating speed, including:

i. means for generating an output signal representing a function of roll spacing in relation to mill speed,

ii. means effective over a range of mill speeds for counteracting said signal, and

iii. means operable selectively when metal strip of the desired exit thickness is obtained at any convenient speed in said range to initiate roll spacing adustments in relation to subsequent changes in mill speed.

6. A rolling mill having opposed work rolls, roll positioning means for adjusting the spacing of said rolls and a control system for actuating said positioning means to effect changes in roll spacing as operation of the mill proceeds, wherein said control system comprises:

a. means for sensing mill speed;

b. means for sensing the force acting on said rolls as a metal strip passes therebetween;

c. means responsive to said force and speed sensing means for generating an output signal representing a program of predetermined roll spacing adjustments;

d. circuit means effective to counteract and essentially null said signal during relatively low speed operation of the mill; and

e. means operable selectively over a range of mill speeds to initiate said program of roll spacing adjustments in response to subsequent changes in mill speed and roll force as the mill is accelerated to normal operating speed.

7. In a rolling mill having opposed work rolls, including support bearings for said work rolls of the type exhibiting a tendency to bring the rolls closer together as the mill speed increases, roll positioning means to adjust the spacing of said rolls and a control system for actuating said positioning means to effect changes in roll spacing as operation of the mill proceeds, the improvement wherein said control system comprises:

a. means for generating an output signal representing a program of predetermined roll spacing adjustments effective substantially to counteract the speed-related behavior of said bearings; and

b. means operable selectively over a range of mill speeds to initiate said roll spacing adjustments in response to subsequent changes in mill speed, including circuit means for counteracting said signal to due to changes in mill speed that occur prior to initiation of said roll spacing adjustments.

8. The rolling mill of claim 7, wherein the speedrelated behavior of said bearings is also dependent on the force acting on the rolls, being represented in graphical form as a family of curves indicating the relationship between oil-film thickness of said bearings and the mill speed for various roll force conditions;

a. said signal generating means being effective to produec an output signal proportional to said oil-film thickness as a function of roll force and mill speed; and

b. said circuit means providing a variable reference of mill speed to establish the starting point of said program of roll spacing adjustments.

9. The rolling mill of claim 8 including means for varying said output signal to cause a greater roll spacing adjustment than that needed to counteract said bearing behavior, thereby compensating for other speed related effects of mill operation.

10. In a rolling mill having opposed work rolls and means for adjusting a selected parameter of the mill to regulate a speed-related aspect of its operation, a control system comprising:

a. means for generating an output signal representing a function of the selected parameter in relation to the mill speed;

b. means effective over a range of mill speeds to counteract said signal, and operable selectively at any speed in that range to produce a modified signal based on subsequent changes in mill speed; and

c. means for actuating said adjusting means in response to the modified signal.

11. In a rolling mill having opposed work rolls for reducing the thickness of a metal workpiece passing therebetween, means for generating an output signal representing a predetermined function of roll spacing in relation to mill speed, and a roll positioning system for changing the roll spacing automatically to compensate substantially for varying thickness of the workpiece as the mill speed varies, the improvement comprising:

a. control circuit means selectively operable in a first or a second mode and responsive to said output signal for applying a modified output signal to said roll positioning system;

b. said control circuit means including means operable in said first mode for producing a modified output signal of zero value, and operable in said second mode for producing a modified output signal having a value equal at any instant to the difference between said output signal at said instant and said output signal at the instant said control circuit means last changed from said first to said second mode;

0. whereby changes in mill speed prior to the last change from said first to said second mode do not effect a roll spacing adjustment.

12. The improvement as claimed in claim I] wherein said control circuit means includes:

a closed loop servo system operable in a first or a second mode;

means for controlling said servo system to operate in said first or said second mode;

said servo system being, responsive to said output signal when in said first mode for generating a compensation signal that varies as said output signal varies, and operable in said second mode for maintaining said compensation signal constant at the value it had at termination of said first mode; and,

means combining said output signal and said compensation signal to produce said modified output signal. 

1. In a rolling mill having opposed work rolls for reducing the thickness of a metal workpiece passing therebetween and a roll positioning system for increasing the roll spacing automatically to compensate substantially for decreasing thickness of the workpiece as the mill speed increases, the improvement comprising: control means which may be selectively rendered operable by an operator at any speed over a range of mill speeds for actuating said roll positioning system, said control means including, means operable before and after said control means is rendered operable for generating an output signal representing a predetermined function of roll spacing in relation to mill speed, and means for modifying said signal to eliminaTe the effect of changes in mill speed previous to the time said control means is rendered operable.
 2. A rolling mill according to claim 1 wherein said signal generating means comprises means for sensing mill speed and amplifier means responsive to said sensing means for generating an output signal representing a function of roll spacing in relation to mill speed.
 3. A rolling mill according to claim 2 wherein said means for modifying the output signal of said amplifier means comprises a closed loop servo system including a motor driven potentiometer and associated circuitry operable selectively to produce a modified signal representing said function of roll spacing relative to changes in mill speed subsequent to the time said closed loop servo system is rendered inoperable.
 4. In a rolling mill having opposed work rolls for reducing the thickness of a metal workpiece passing therebetween, including roll positioning means for adjusting the spacing between said rolls and a control system for causing programmed changes in roll spacing as a function of roll force and mill speed, the improvement wherein said control system comprises the combination of: a. a load cell for sensing the force acting on said rolls; b. a force amplifier unit responsive to said load cell for generating an output signal proportional to the roll force; c. means for sensing mill speed; d. a speed amplifier unit responsive to said speed sensing means for generating an output signal representing a predetermined function of roll spacing in relation to mill speed; e. circuit means operable selectively in response to said output signal of the speed amplifier over a range of mill speeds to produce a modified signal representing that portion of the speed amplifier output caused by changes in mill speed subsequent to the time said circuit means is made operable; and f. an analog-digital converter responsive to said modified signal and the output signal of said force amplifier for generating a position signal to actuate said roll positioning means.
 5. In a rolling mill having opposed work rolls for acting on a metal workpiece passing therebetween, the thickness of said workpiece as it leaves the rolls being affected by the mill speed, becoming progressively thinner as mill speed increases, the combination of: a. roll positioning means for adjusting the spacing of said rolls during operation of the mill; and b. control means for actuating said positioning means to cause roll spacing adjustments effective to keep exit thickness of the metal substantially constant as the mill is accelerated to normal operating speed, including: i. means for generating an output signal representing a function of roll spacing in relation to mill speed, ii. means effective over a range of mill speeds for counteracting said signal, and iii. means operable selectively when metal strip of the desired exit thickness is obtained at any convenient speed in said range to initiate roll spacing adustments in relation to subsequent changes in mill speed.
 6. A rolling mill having opposed work rolls, roll positioning means for adjusting the spacing of said rolls and a control system for actuating said positioning means to effect changes in roll spacing as operation of the mill proceeds, wherein said control system comprises: a. means for sensing mill speed; b. means for sensing the force acting on said rolls as a metal strip passes therebetween; c. means responsive to said force and speed sensing means for generating an output signal representing a program of predetermined roll spacing adjustments; d. circuit means effective to counteract and essentially null said signal during relatively low speed operation of the mill; and e. means operable selectively over a range of mill speeds to initiate said program of roll spacing adjustments in response to subsequent changes in mill speed and roll force as the mill is accelerated to normal operating speed.
 7. In a rolling mill having opposed work rolls, including support bearings for said work rolls of the type exhibiting a tendency to bring the rolls closer together as the mill speed increases, roll positioning means to adjust the spacing of said rolls and a control system for actuating said positioning means to effect changes in roll spacing as operation of the mill proceeds, the improvement wherein said control system comprises: a. means for generating an output signal representing a program of predetermined roll spacing adjustments effective substantially to counteract the speed-related behavior of said bearings; and b. means operable selectively over a range of mill speeds to initiate said roll spacing adjustments in response to subsequent changes in mill speed, including circuit means for counteracting said signal to due to changes in mill speed that occur prior to initiation of said roll spacing adjustments.
 8. The rolling mill of claim 7, wherein the speed-related behavior of said bearings is also dependent on the force acting on the rolls, being represented in graphical form as a family of curves indicating the relationship between oil-film thickness of said bearings and the mill speed for various roll force conditions; a. said signal generating means being effective to produce an output signal proportional to said oil-film thickness as a function of roll force and mill speed; and b. said circuit means providing a variable reference of mill speed to establish the starting point of said program of roll spacing adjustments.
 9. The rolling mill of claim 8 including means for varying said output signal to cause a greater roll spacing adjustment than that needed to counteract said bearing behavior, thereby compensating for other speed related effects of mill operation.
 10. In a rolling mill having opposed work rolls and means for adjusting a selected parameter of the mill to regulate a speed-related aspect of its operation, a control system comprising: a. means for generating an output signal representing a function of the selected parameter in relation to the mill speed; b. means effective over a range of mill speeds to counteract said signal, and operable selectively at any speed in that range to produce a modified signal based on subsequent changes in mill speed; and c. means for actuating said adjusting means in response to the modified signal.
 11. In a rolling mill having opposed work rolls for reducing the thickness of a metal workpiece passing therebetween, means for generating an output signal representing a predetermined function of roll spacing in relation to mill speed, and a roll positioning system for changing the roll spacing automatically to compensate substantially for varying thickness of the workpiece as the mill speed varies, the improvement comprising: a. control circuit means selectively operable in a first or a second mode and responsive to said output signal for applying a modified output signal to said roll positioning system; b. said control circuit means including means operable in said first mode for producing a modified output signal of zero value, and operable in said second mode for producing a modified output signal having a value equal at any instant to the difference between said output signal at said instant and said output signal at the instant said control circuit means last changed from said first to said second mode; c. whereby changes in mill speed prior to the last change from said first to said second mode do not effect a roll spacing adjustment.
 12. The improvement as claimed in claim 11 wherein said control circuit means includes: a closed loop servo system operable in a first or a second mode; means for controlling said servo system to operate in said first or said second mode; said servo system being responsive to said output signal when in said first mode for generating a compensation signal that varies as said output signal varies, and operablE in said second mode for maintaining said compensation signal constant at the value it had at termination of said first mode; and, means combining said output signal and said compensation signal to produce said modified output signal. 